Adaptive plasticity is a phrase used to explain how cancer cells gain selective growth and survival capabilities through phenotypic changes in response to their environment. Adaptive plasticity is exemplified by epithelial to mesenchymal transition (EMT) a process, which enables cancer cells to become migratory and invasive as well as having an increase in survival properties. This process can be reversed to a mesenchymal to epithelial transition (MET), which is believed to favor tumor engraftment and growth of cancer cells at metastatic sites. EMT in cancer cells is linked with isoform switching of the transmembrane protein CD44 from expressing the CD44 variant isoform (CD44v) to expressing CD44 standard form (CD44s). In this context we found that treatment of BxPC3 pancreatic ductal adenocarcinoma (PDAC) cell line with gemcitabine induces EMT and a high level of expression of CD44s with a decrease in expression of CD44v isoforms. This suggests that an adverse effect of chemotherapy may drive EMT, enhancing invasiveness and rendering tumor cells more resistant to chemotherapy. Moreover, we were able to use flow cytometry to select out from a PDAC cell lines (CFPAC-1 and AsPC1) a subpopulation of cells expressing high levels of CD44s. Knockdown of CD44s, in these cells, caused them to undergo MET, to be less invasive and to become more responsive to chemotherapy. These studies suggest that chemotherapy or other environmental stresses may induce a CD44s phenotype that promotes survival and is more invasive; however optimal tumor regrowth following chemotherapy or growth after engraftment of micrometastases may require re-switching to a CD44v phenotype and down regulation of CD44s expression. We hypothesize that CD44s and its variant isoforms are key regulators of adaptive plasticity in pancreatic cancer cells. We further hypothesize that understanding the molecular basis for adaptive plasticity will provide new strategies for improving therapy. These studies represent new data from our laboratory and build on 20 years of research solely focused on the biology and treatment of pancreatic cancer. To address these hypotheses we propose the following three objectives. Objective 1. Determine the functional relevance of CD44s and CD44v in PDAC. This objective will be addressed using isogenic matched cell line models in which expression of CD44s and CD44v isoforms are regulated. The effects on modulating expression of CD44v and CD44s will be assessed for phenotype, invasiveness, response to chemotherapy, tumor engraftment and growth. Objective 2. Determine the mechanism(s) that regulates CD44 isoform switching and CD44 expression. This objective will determine the potential role of EMT related transcriptional repressors Snail and Zeb1 in suppressing ESRP1 that is required for expression of CD44v isoforms and further identify the mechanisms that lead to the increased transcription of CD44s. Objective 3. Determine whether anti-CD44 antibody prevents CD44 isotype switching or expression level and whether this improves response to chemotherapy and reduces metastasis. These studies will provide important new insights related to adaptive plasticity and determine whether this knowledge can be used to counter act adverse effects of chemotherapy induced phenotypic switching. Inhibiting or reversing the switch towards an EMT phenotype may prove to be an important strategy for increasing the response to chemotherapy and patient survival.